1. Field of the Invention
The invention relates to a light emitting diode (LED) driving apparatus, more particularly, to an LED driving apparatus having features of expandability and current balance/current matching.
2. Description of Related Art
In recent years, with development of semiconductor technology, portable electronic produces and flat panel displays are quickly developed, and in various flat panel displays, a liquid crystal display (LCD) becomes popular in the market due to its advantages of low operating voltage, no irradiation, light weight and small size, etc. Generally, since the LCD panel has no luminescent function itself, a backlight module is required to be disposed under the LCD panel for providing a backlight source to the LCD panel.
The conventional backlight modules are grouped into cold cathode fluorescent lamp (CCFL) backlight modules and light emitting diode (LED) backlight modules. Since the LED backlight module can improve a color gamut of the LCD, the LED backlight modules are generally used to replace the CCFL backlight modules.
The LED backlight module has a plurality of LED strings arranged in parallel, and each of the LED strings consists of a plurality of LEDs connected in series. In an actual application, a DC-DC converter is generally used to convert an input power into a DC voltage for simultaneously driving each of the LED strings. However, since a load feature (I-V curve) of each of the LED strings is different, a current flowing through each of the LED strings is different (i.e. current imbalance), so that not only a light source brightness provided to the LCD panel by the LED backlight module is uneven, but also a service life of the LEDs is shortened.
To resolve such problem, a current balance or current matching control circuit can be added to the LED backlight module to regulate the current flowing through each of the LED strings, so that the current flowing through each of the LED strings can be the same (i.e. current balance/current matching) The commonly used current balance or current matching control circuit may consist of a current feedback compensation circuit or a current mirror circuit formed by a plurality of active devices (for example, metal-oxide-semiconductor field-effect transistors (MOSFETs)). Moreover, the current balance or current matching control circuit may also consist of current regulation chips (for example, current sink integrated circuits (ICs)).
However, a number of regulation channels of the current regulation chip is generally fixed (for example, four or six regulation channels), and since one regulation channel is used for regulating the current flowing through one LED string, when the LED backlight module has ten LED strings (the number is determined according to different panel sizes), three current regulation chips each having four regulation channels or two current regulation chips each having six regulation channels have to be used, so that two regulation channels are useless in both cases, which may cause a waste of cost.